1. Field of the Invention
The invention relates to flat antennas for land based signals and radio connections in the Ghz range. In particular, the invention relates to mobile radio transmission in the 900 MHz and 1.8 GHz-bands.
2. The Prior Art
The invention is based on an operating principle disclosed in part in German patent application No. 1 95 045 77. Essentially, the prior art discloses two flat elements made of electrically conductive material aligned parallel to each other. These flat elements are connected by a lateral short circuit. One of the flat elements is a ground plane or reference surface and is connected to an external conductor, coupled to a coaxial cable, while the other element is a "surface segment" and is connected to the center conductor of the coaxial cable. The contours and dimensions of the surface segment determine its function. The elements, when coupled together, form a cavity resonator during operation. During resonance, there is a field buildup in the open lateral zones, causing a radial radiation pattern of a large order of magnitude. If the ground plane is sufficiently larger than the surface segment connected to the coaxial center conductor, or if it is directly positioned above a larger conductive surface, then a radiation pattern is produced similar to that of a monopole.
In addition, German Patent Application No. 1 96 14 068 discloses a flat antenna with a ground plane having sides that arch towards the edge in one coordinate. For example, if the ground plane is designed in the form of a flat trough or cap-shaped surface, the dimensions of the ground plane can be reduced without impairing the function of the antenna and its radiation parameters.
Extensive tests reveal that good antenna gains are achieved by changing the design of the ground plane. For example, if the ground plane is redesigned so that it arches on both sides toward the edge in at least one coordinate, then these better gains can be achieved. Another way is to redesign the ground plane in the form of a flat trough or cap shaped surface, where the dimensions of the ground plane can be reduced without impairing the function of the antenna and its radiation parameters.
The antenna principle described above has one drawback in that it requires approximately symmetrical conditions when two antenna components are mounted, i.e. the antenna surface segment has to be disposed above the ground plane as centrally as possible. This requirement leads problems when these antennas are mounted on automobiles.
The flat antenna can be mounted on a vehicle in one of two ways. It can be mounted either as an external attachment part on the outside of the vehicle, or on the interior of the vehicle below the front or rear windshield. If the antenna is mounted on the rear window, then the dimensions of the ground plane are reduced using a spherical design. This design is referred to above in part in German Patent Application 196 14 068. However, this design is insufficient since the rear window in many cases is slanted off from the horizontal plane, so that the radiation characteristic of the antenna appears as a circle displaced in the direction of the rear of the vehicle. Thus, the radiation level is somewhat weaker in the direction of the front of the vehicle.
Therefore, the invention provides a way of improving the operating parameters of flat antennas to allow for different installation positions, and for greater flexibility.